1. Field of the Invention
The present invention relates to a recording head and a recording apparatus, executing recording by discharging a liquid droplet.
2. Description of the Related Art
Recording apparatuses for executing a recording on a recording medium such as a paper, a cloth, a plastic sheet or an OHP sheet are known in various recording systems, such as a wire dot recording system, a thermal recording system, a thermal transfer recording system and an ink jet recording system.
Among these recording systems, the ink jet recording system utilizes discharging ink, according to recording information, from a fine hole for discharging ink (hereinafter called a discharge port), provided in an ink jet recording head (hereinafter simply called a recording head).
In an ink discharging operation in the recording head, pressure generated for the ink discharge propagates, through the ink in a flow path, in a direction toward the discharge port and in a direction toward a liquid chamber, that serves as an ink supply source to the flow path. By the function of pressure propagating toward the discharge port, the ink in the flow path is pushed out from the discharge port, thereby forming a flying liquid droplet.
When the ink leaves the discharge port as a liquid droplet, a meniscus formed in the flow path in the vicinity of the discharge port is retracted according to the amount of the discharged liquid droplet. Then, by an action of pulling back the meniscus toward the discharge port, the ink filling state in the flow path returns to a state prior to the discharge after the lapse of a certain time. This phenomenon is called “refilling”, and, in the actual recording, the above-described operations are repeated and stable ink droplet discharges are obtained in a continuous manner by satisfactory refilling.
However, there may result a situation where the refilling is not executed in time for the next discharge, in relation for example with a discharge frequency. In the case that the ink discharge is executed in a state where the refilling is incomplete, there may result a discharge failure such as a decrease in the amount of the discharged ink droplet. As a result, a diameter of an ink dot, formed by the discharged ink droplet on the recording medium, decreases to result in a deterioration in the overall recording quality. Also the precision of the landing point of the discharged ink droplet on the recording medium may be deteriorated to induce defects such as blurring, a deviated recording, streaking, and white spots in the recorded image.
The above-described problems in the recording technology utilizing a liquid such as the ink jet recording system have been tried to be solved by an improvement in the structure such as the flow path or by adjusting the physical properties of the ink. However, in a recording head in which a plurality of discharge ports are arrayed, it is often not possible to obtain a sufficient effect by this structural improvement or adjustment. These problems will be described in the following, with reference to the accompanying drawings.
FIGS. 12A and 12B are views illustrating pressure resulting from the ink discharge in a direction toward the discharge port, and FIGS. 13A and 13B are views for describing the pressure necessary for obtaining a satisfactory refilling state. FIGS. 12A and 13A are plan views of a principal part of the recording head, and FIGS. 12B and 13B are cross-sectional views of the principal part seen from a discharge direction of the ink.
A recording head 100 includes a plurality of discharge ports (abbreviated in drawing), flow paths 102 respectively communicating with these discharge ports, a discharge energy generating element 103 disposed in each flow path 102, and a supply opening 104 for supplying the flow paths with the ink. The supply opening 104 communicates with an unillustrated ink tank (also called ink cartridge) through an ink supply path 105, whereby the supply opening 104 is constantly filled with the ink.
As illustrated in FIGS. 12A and 12B, in the case that the ink is discharged from a number of discharge ports at the same time or with a certain time difference, the pressure generated by the ink discharge in each flow path 102 propagates from each flow path 102 toward the supply opening 104. Such pressures are united in the supply opening 104 and become a single large pressure. The pressure generated in each flow path 102 functions as a force for pushing back the ink toward the supply opening 104 as indicated by an arrow A, and the sum of these forces becomes several times greater than in a recording head for example having only one discharge port.
In this case, in order to obtain a satisfactory refilling state, it is necessary, as illustrated in FIGS. 13A and 13B, to move the ink rapidly and in a large amount toward the discharge port (as indicated by arrows B). In order to realize this change in the moving direction of the ink, a pressure capable of overcoming the initial large inertial force (total pressure) of the ink is required.
However, the capillary force of ink, realizing the refilling in each flow path 102, is not sufficient for displacing a large amount of ink instantaneously toward the discharge port in opposition to the above mentioned total pressure toward the supply opening 104. Consequently, with an increase in the initial inertial force in the above mentioned ink displacement, a longer time is required for restoring the meniscus 106. Therefore, when the discharge frequency is lowered in order to obtain a sufficient time for the returning of the meniscus, the recording speed becomes lowered. On the other hand, when a sufficient time for the returning of the meniscus cannot be obtained, for example the discharged ink droplet cannot be obtained in a prescribed liquid amount, so that satisfactory recording is hindered. This phenomenon is known to be particularly conspicuous in an initial period of recording after starting of recording.
FIGS. 14A and 14B are views illustrating the mechanism of the above-described phenomenon, FIG. 14A illustrates curves indicating retraction of the meniscus, and FIG. 14B is a view illustrating a schematic constitution of the discharge port and the vicinity thereof.
In FIG. 14A, a retraction amount L [μm] of the meniscus indicated on the ordinate is represented, illustrated in FIG. 14B, by a distance L from an end portion of the discharge port 101 to the meniscus. More specifically, it corresponds to the distance from the discharge port 101 to the most retracted portion of the ink meniscus.
A curve CM1 in FIG. 14A, indicating the change in time of the meniscus retraction for example in a recording head having only one discharge port, indicates the following facts. After a certain period from a time t0 at which the energy from the discharge energy generating element 103 is applied to the ink in the flow path 102, the meniscus 106 formed in the vicinity of the ink discharge port of the flow path 102 starts to retract rapidly from a time t0′. Stated differently, the meniscus 106 starts to retract from a time when an ink discharge is executed. The amount of retraction, reaching maximum at a time t1′, is relatively large. Thereafter, by the action of a returning force by the capillary force, the meniscus 106 starts to return to the original position, and the refilling is completed at a time t1.
On the other hand, in case of a recording head having a plurality of discharge ports, as represented by a curve CM2, the maximum retraction amount at the time t1′ is smaller than in the above-mentioned case, but the refilling speed is lower as indicated by an ending time t2.
This is presumably because, as described above, the total sum of the pressures from the plural flow paths 102 to press the ink backwards significantly exceeds a pressure for causing an ink flow in the supply opening 104. More specifically, an excessive pressure, exceeding the pressure for causing an ink flow in the supply opening 104 acts on the ink, thereby extremely retarding, in an initial period, the refilling speed for returning the meniscus 106.
After the discharges are repeated in succession, the phenomenon described above infrequently occurs, since a stationary flow of ink is formed from the ink supply path 105 (cf. FIGS. 12A to 13B) to the supply opening 104. In fact, the above-described phenomenon occurs evidently in an initial period of the discharges, particularly until the discharging operation is repeated about 200 times thereby forming a stationary flow of the ink.
In a recording head having a plurality of discharge ports, the decrease in the refilling speed does not become a problem when a period of applications of print signals to the discharge energy generating element is selected equal to or longer than a period from the time t0 to t2 illustrated in FIG. 14A.
However, when the next signal is applied, for the purpose of a high-speed recording, with a period shorter than the period from the time t0 to t2, namely before the completion of refilling, there may result for example a decrease in the amount of discharged ink droplets whereby a satisfactory recording may become impossible. Stated differently, when a next signal is applied in a state where the retraction amount L of the meniscus is 30 μm or more, a decrease in the amount of the discharged ink droplets may result, whereby a satisfactory recording may become impossible.
In order to solve these problems, Japanese Patent Application Laid-Open No. H06-210872 discloses a construction having an air chamber (buffer chamber) at a rear side, opposite to the nozzle side with respect to the supply opening in the head unit. More specifically, in the disclosed construction, a buffer chamber is formed in a position close to the nozzle (array) to alleviate vibration (high-frequency vibration) of the liquid caused by the driving, bubbling, and discharging in an individual nozzle, thereby not detrimentally affecting other nozzles. Thus, Japanese Patent Application Laid-Open No. H06-210872 proposes to solve the above mentioned problems by preventing the crosstalk by the above-described construction.
Japanese Patent Application Laid-Open No. H06-210872 also discloses a construction of forming, in a path from the ink tank portion to the head portion, a buffer chamber which is formed in the head unit, in the ink supply tube for ink supply thereto, and in the connecting part of the two. In particular, FIGS. 12A and 12B illustrate a construction having a buffer chamber around the supply tube of a constant cross section.
In order to reduce the crosstalk by this buffer chamber, it is desirable that there should be a large number of buffer chambers in the vicinity of the nozzle array. In consideration of the structure of the recording head, a side face of the ink supply opening at the rear side of the recording element substrate or of the ink supply opening of an alumina base plate supporting the recording element substrate is the best position for forming the buffer chamber.
Japanese Patent Application Laid-Open No. 2001-130004 discloses a construction of providing a buffer chamber on the side face of the ink supply opening of an alumina base plate. More specifically, around the ink supply opening on the surface of an alumina base plate on a surface thereof adhered with the recording element (chip), a hole communicating with the ink supply opening is formed in plural. It is further disclosed to form the buffer chamber by covering an upper part of this hole, by adhesion with the recording element.
However, the formation of the buffer chamber as disclosed in Japanese Patent Application Laid-Open No. 2001-130004 is associated with the following problem. When the buffer chamber is formed in the above mentioned adhering position with the chip, the area of adhesion (contact area) with the chip becomes smaller. In a recording element executing ink discharge particularly by the bubble jet system, the chip itself is heated by the heater, and the heat accumulated in the chip is dissipated through the contact area with the base plate. Therefore, when the contact area with the chip is made smaller by the formation of the buffer chamber, the speed of heat dissipation is reduced leading to problematical printing. In particular, recent recording elements, called for higher print resolution, higher print speed and a longer nozzle array, tend to cause problems in the printing, conspicuously as a consequence of the lowered heat dissipation speed.
Also recent developments are directed actively toward a compact recording element (chip) itself as a result of size reduction in the printer itself and in the recording head. For example, a method has already been devised to dispose plural nozzle arrays for different colors on a single recording element substrate, with a distance between the nozzle arrays as small as possible. However, in the construction of the buffer chamber disclosed in Japanese Patent Application Laid-Open No. 2001-130004, the buffer chamber becomes difficult to be disposed or is limited in the position thereof, by the presence of the adjacent nozzle.
Furthermore, the disposition of the buffer chamber disclosed in Japanese Patent Application Laid-open No. 2001-130004 is difficult to apply to a full multi-head or a multi-array nozzle head. Full multi-head means a recording head that has a nozzle array length of the recording element corresponding to the width of a recording sheet and that is used in a recording method in which the recording sheet is conveyed immediately under the recording head and is thus recorded. Also, multi-array nozzle head means a recording head including plural units of the nozzle array of a full-multi head.